That's one of one of the main use cases I had in mind when I wrote this: https://github.com/ryancdotorg/threshcrypt - you encrypt a file using N passwords and require that at least M passwords be presented in order to decrypt. At the moment it's not suitable for non-technical users, though you could make a bootable thumb drive that autoruns it and saves the decrypted file. I was also able to embed it in an initramfs on Debian for use with full disk encryption.